Glial activation in a pilocarpine rat model for epileptogenesis: A morphometric and quantitative analysis

CDDO-Me Distinctly Regulates Regional Specific Astroglial Responses to Status Epilepticus via ERK1/2-Nrf2, PTEN-PI3K-AKT and NFκB Signaling Pathways

2-Cyano-3,12-dioxo-oleana-1,9(11)-dien-28-oic acid methyl ester (CDDO-Me) is a triterpenoid analogue of oleanolic acid. CDDO-Me shows anti-inflammatory and neuroprotective effects. Furthermore, CDDO-Me has antioxidant properties, since it activates nuclear factor-erythroid 2-related factor 2 (Nrf2), which is a key player of redox homeostasis. In the present study, we evaluated whether CDDO-Me affects astroglial responses to status epilepticus (SE, a prolonged seizure activity) in the rat hippocampus in order to understand the underlying mechanisms of reactive astrogliosis and astroglial apoptosis. Under physiological conditions, CDDO-Me increased Nrf2 expression in the hippocampus without altering activities (phosphorylations) of phosphatase and tensin homolog deleted on chromosome 10 (PTEN), phosphatidylinositol-3-kinase (PI3K), and AKT. CDDO-Me did not affect seizure activity in response to pilocarpine. However, CDDO-Me ameliorated reduced astroglial Nrf2 expression in the CA1 region and the molecular layer of the dentate gyrus (ML), and attenuated reactive astrogliosis and ML astroglial apoptosis following SE. In CA1 astrocytes, CDDO-Me inhibited the PI3K/AKT pathway by activating PTEN. In contrast, CDDO-ME resulted in extracellular signal-related kinases 1/2 (ERK1/2)-mediated Nrf2 upregulation in ML astrocytes. Furthermore, CDDO-Me decreased nuclear factor-κB (NFκB) phosphorylation in both CA1 and ML astrocytes. Therefore, our findings suggest that CDDO-Me may attenuate SE-induced reactive astrogliosis and astroglial apoptosis via regulation of ERK1/2-Nrf2, PTEN-PI3K-AKT, and NFκB signaling pathways.

Normal and epilepsy-associated pathologic function of the dentate gyrus

The dentate gyrus plays critical roles both in cognitive processing, and in regulation of the induction and propagation of pathological activity. The cellular and circuit mechanisms underlying these diverse functions overlap extensively. At the cellular level, the intrinsic properties of dentate granule cells combine to endow these neurons with a fundamental reluctance to activate, one of their hallmark traits. At the circuit level, the dentate gyrus constitutes one of the more heavily inhibited regions of the brain, with strong, fast feedforward and feedback GABAergic inhibition dominating responses to afferent activation. In pathologic states such as epilepsy, a number of alterations within the dentate gyrus combine to compromise the regulatory properties of this circuit, culminating in a collapse of its normal function. This epilepsy-associated transformation in the fundamental properties of this critical regulatory hippocampal circuit may contribute both to seizure propensity, and cognitive and emotional comorbidities characteristic of this disease state.

Serial Quantitative TSPO-Targeted PET Reveals Peak Microglial Activation up to 2 Weeks After an Epileptogenic Brain Insult

Experimental and clinical evidence suggests that neuroinflammation, triggered by epileptogenic insults, contributes to seizure development. We used translocator protein-targeted molecular imaging to obtain further insights into the role of microglial activation during epileptogenesis.

METHODS

As epileptogenic insult, a status epilepticus (SE) was induced in rats by lithium pilocarpine. Rats were subjected to (11)C-PK11195 PET scans before SE; at 4 h after SE; at 1, 2, 5, 7, 14, and 22 d after SE; and at 14-16 wk after SE. For data evaluation, brain regions were outlined by coregistration with a standard rat brain atlas, and percentage injected dose/cm(3) and binding potential (simplified reference tissue model with cerebellar gray matter as a reference region) were calculated. For autoradiography and immunohistochemical evaluation, additional rats were decapitated without prior SE or 2, 5, or 14 d after SE.

RESULTS

After SE, increases in (11)C-PK11195 uptake and binding potential were evident in epileptogenesis-associated brain regions, such as the hippocampus, thalamus, or piriform cortex, but not in the cerebellum beginning at 2-5 d and persisting at least 3 wk after SE. Maximal regional signal was observed at 1-2 wk after SE. Autoradiography confirmed the spatiotemporal profile. Immunohistochemical evaluation revealed microglial and astroglial activation as well as neuronal cell loss in epileptogenesis-associated brain regions at all investigated time points. The time course of microglial activation was consistent with that demonstrated by tracer techniques.

CONCLUSION

Translocator protein-targeted PET is a reliable tool for identifying brain inflammation during epileptogenesis. Neuroinflammation mainly affects brain regions commonly associated with seizure generation and spread. Definition of the time profile of neuroinflammation may facilitate the development of inflammation-targeted, antiepileptogenic therapy.

Subcortical heterotopia appearing as huge midline mass in the newborn brain

INTRODUCTION

We report the case of a 2-year-old boy who showed a huge midline mass in the brain at prenatal assessment.

CASE REPORT

After birth, magnetic resonance imaging (MRI) revealed a conglomerate mass with an infolded microgyrus at the midline, which was suspected as a midline brain-in-brain malformation. MRI also showed incomplete cleavage of his frontal cortex and thalamus, consistent with lobar holoprosencephaly. The patient underwent an incisional biopsy of the mass on the second day of life. The mass consisted of normal central nervous tissue with gray and white matter, representing a heterotopic brain. The malformation was considered to be a subcortical heterotopia. With maturity, focal signal changes and decreased cerebral perfusion became clear on brain imaging, suggesting secondary glial degeneration. Coincident with these MRI abnormalities, the child developed psychomotor retardation and severe epilepsy focused on the side of the intracranial mass.

Long-term effects of pre and post-ischemic exercise following global cerebral ischemia on astrocyte and microglia functions in hippocampus from Wistar rats

Persistent effects of pre- and postischemic exercise on glial cells activation after global cerebral ischemia remains poorly understood. Here, we investigated the effect of both pre and postischemic treadmill exercise protocols (20min/day during 2 weeks) on glial cells immunostaining in the hippocampus of Wistar rats submitted to global ischemia. A synergistic effect between ischemia and postischemic exercise on the astrocytic area was demonstrated. Postischemic exercise partially reversed the ischemia-induced increase on the area occupied by microglia, without any effect of pre-ischemic protocol. In conclusion, postischemic exercise distinctly modulates astrocyte and microglia immunostaining in the hippocampal dentate gyrus following global cerebral ischemia in Wistar rats.

Effects of traumatic brain injury on reactive astrogliosis and seizures in mouse models of Alexander disease

Alexander disease (AxD) is the only known human pathology caused by mutations in an astrocyte-specific gene, glial fibrillary acidic protein (GFAP). These mutations result in abnormal GFAP accumulations that promote seizures, motor delays and, ultimately, death. The exact contribution of increased, abnormal levels of astrocytic mutant GFAP in the development and progression of the epileptic phenotype is not clear, and we addressed this question using two mouse models of AxD. Comparison of brain seizure activity spontaneously and after traumatic brain injury (TBI), an effective way to trigger seizures, revealed that abnormal GFAP accumulation contributes to anomalous brain activity (increased non-convulsive hyperactivity) but is not a risk factor for the development of epilepsy after TBI. These data highlight the need to further explore the complex and heterogeneous response of astrocytes towards injury and the involvement of GFAP in the progression of AxD.

Electroacupuncture at ST36-ST37 and at ear ameliorates hippocampal mossy fiber sprouting in kainic acid-induced epileptic seizure rats

Our previous study showed that mossy fiber sprouting can occur in the hippocampus region in rats 6 wk after kainic acid-induced epileptic seizure, and this mossy fiber sprouting can facilitate epileptogenesis. Transcutaneous auricular vagal nerve stimulation (VNS), which is similar to cervical VNS, can reduce the occurrence of epileptic seizure in intractable epilepsy patients. Greater parasympathetic nerve activity can be caused by 2 Hz electroacupuncture (EA). Therefore, we investigated the effect of 2 Hz EA at ST-36-ST37 and at the ear on mossy fiber sprouting in kainic-treated Sprague-Dawley rats. The results indicated that applying 2 Hz EA at ST36-ST37 and at the ear for 3 d per week over 6 consecutive weeks can ameliorate mossy fiber sprouting in the hippocampus region of rats. These results indicated that applying 2 Hz EA at ST36-ST37 and at the ear might be beneficial for the treatment and prevention of epilepsy in humans.

Widespread activation of microglial cells in the hippocampus of chronic epileptic rats correlates only partially with neurodegeneration

Activation of microglial cells (brain macrophages) soon after status epilepticus has been suggested to be critical for the pathogenesis of mesial temporal lobe epilepsy (MTLE). However, microglial activation in the chronic phase of experimental MTLE has been scarcely addressed. In this study, we questioned whether microglial activation persists in the hippocampus of pilocarpine-treated, epileptic Wistar rats and to which extent it is associated with segmental neurodegeneration. Microglial cells were immunostained for the universal microglial marker, ionized calcium-binding adapter molecule-1 and the activation marker, CD11b (also known as OX42, Mac-1). Using quantitative morphology, i.e., stereology and Neurolucida-based reconstructions, we investigated morphological correlates of microglial activation such as cell number, ramification, somatic size and shape. We find that microglial cells in epileptic rats feature widespread, activation-related morphological changes such as increase in cell number density, massive up-regulation of CD11b and de-ramification. The parameters show heterogeneity in different hippocampal subregions. For instance, de-ramification is most prominent in the outer molecular layer of the dentate gyrus, whereas CD11b expression dominates in hilus. Interestingly, microglial activation only partially correlates with segmental neurodegeneration. Major neuronal death in the hilus, CA3 and CA1 coincides with strong up-regulation of CD11b. However, microglial activation is also observed in subregions that do not feature neurodegeneration, such as the molecular and granular layer of the dentate gyrus. This in vivo study provides solid experimental evidence that microglial cells feature widespread heterogeneous activation that only partially correlates with hippocampal segmental neuronal loss in experimental MTLE.

Neonatal maternal separation up-regulates protein signalling for cell survival in rat hypothalamus

We have previously reported that in response to early life stress, such as maternal hyperthyroidism and maternal separation (MS), the rat hypothalamic vasopressinergic system becomes up-regulated, showing enlarged nuclear volume and cell number, with stress hyperresponsivity and high anxiety during adulthood. The detailed signaling pathways involving cell death/survival, modified by adverse experiences in this developmental window remains unknown. Here, we report the effects of MS on cellular density and time-dependent fluctuations of the expression of pro- and anti-apoptotic factors during the development of the hypothalamus. Neonatal male rats were exposed to 3 h-daily MS from postnatal days 2 to 15 (PND 2-15). Cellular density was assessed in the hypothalamus at PND 21 using methylene blue staining, and neuronal nuclear specific protein and glial fibrillary acidic protein immunostaining at PND 36. Expression of factors related to apoptosis and cell survival in the hypothalamus was examined at PND 1, 3, 6, 9, 12, 15, 20 and 43 by Western blot. Rats subjected to MS exhibited greater cell-density and increased neuronal density in all hypothalamic regions assessed. The time course of protein expression in the postnatal brain showed: (1) decreased expression of active caspase 3; (2) increased Bcl-2/Bax ratio; (3) increased activation of ERK1/2, Akt and inactivation of Bad; PND 15 and PND 20 were the most prominent time-points. These data indicate that MS can induce hypothalamic structural reorganization by promoting survival, suppressing cell death pathways, increasing cellular density which may alter the contribution of these modified regions to homeostasis.

Hippocampal CA field neurogenesis after pilocarpine insult: The hippocampal fissure as a neurogenic niche

Pilocarpine model for temporal lobe epilepsy has shown aberrant neurogenesis, but mainly restricted to the dentate gyrus (DG). Herein, by using a modified protocol, combining pilocarpine with ipratropium bromide, we unexpectedly observed a heretofore-unrecognized distinct cellular population expressing the neuroprogenitor marker doublecortin (DCX) on post insult days (PID) 10, 14 and 18, mainly located in the temporal segment of the hippocampal fissure (hf). Some of these DCX+ cells possessed high morphological complexity and seemed to disperse toward the CA fields. Next, we injected bromodeoxyuridine (BrdU) in early (PID 2-4) and delayed (PID 5-7) fashions and killed the rats 7-35 days later for immunohistochemical and anatomical analysis. Massive increase of BrdU labeling was found in the delayed group and the neural stem cell-specific marker nestin was highly expressed in the same narrow band on PID7, so was glial fibrillary acidic protein (GFAP). Using double labeling with BrdU and a mature neuron marker NeuN, we found discrete but clear BrdU+/NeuN+ double labeled cells in the Cornu Ammonis (CA) pyramidal cell layer on PID35. Based on immunohistochemical and anatomical observations, as well as time-course analysis of BrdU, nestin, GFAP, DCX and NeuN expressions in this population of cells located in/near hf, we wish to suggest that this structure harbors neurogenic niches, in addition of the possible dispersion of neuroprogenitors from subgranular niches to CA fields also revealed by this study. Our results support the few previous reports demonstrating hippocampal CA field neurogenesis in adult rats. Mechanistic basis of the phenomenon is discussed.

Enriched environment induces beneficial effects on memory deficits and microglial activation in the hippocampus of type 1 diabetic rats

Type 1 diabetes mellitus (T1DM) has been associated with long-term complications in the central nervous system, causing brain cellular dysfunctions and cognitive deficits. On the other hand, enriched environment (EE) induces experience-dependent plasticity, especially in the hippocampus, improving the performance of animals in learning and memory tasks. Thus, our objective was to investigate the influence of the EE on memory deficits, locomotion, corticosterone levels, synaptophysin (SYP) protein immunoreactivity, cell survival and microglial activation in the dentate gyrus (DG) of T1DM rat hippocampus. Male Wistar rats (21-day-old) were exposed to EE or maintained in standard housing (controls, C) for 3 months. At adulthood, the C and EE animals were randomly divided and diabetes was induced in half of them. All the animals received 4 doses of BrdU, 24 h apart. Hippocampus-dependent spatial memory, general locomotion and serum corticosterone levels were evaluated at the end of the experiment. The animals were transcardially perfused 30 days post-BrdU administration. Our results showed that EE was able to prevent/delay the development of memory deficits caused by diabetes in rats, however it did not revert the motor impairment observed in the diabetic group. SYP immunoreactivity was increased in the enriched healthy group. The EE decreased the serum corticosterone levels in diabetic adult rats and attenuated the injurious microglial activation, though without altering the decrease of the survival cell. Thus, EE was shown to help to ameliorate cognitive comorbidities associated with T1DM, possibly by reducing hyperactivity in the hypothalamic-pituitary-adrenal axis and microglial activation in diabetic animals.

Gabapentin administration reduces reactive gliosis and neurodegeneration after pilocarpine-induced status epilepticus

The lithium-pilocarpine model of epilepsy reproduces in rodents several features of human temporal lobe epilepsy, by inducing an acute status epilepticus (SE) followed by a latency period. It has been proposed that the neuronal network reorganization that occurs during latency determines the subsequent appearance of spontaneous recurrent seizures. The aim of this study was to evaluate neuronal and glial responses during the latency period that follows SE. Given the potential role of astrocytes in the post-SE network reorganization, through the secretion of synaptogenic molecules such as thrombospondins, we also studied the effect of treatment with the α2δ1 thrombospondin receptor antagonist gabapentin. Adult male Wistar rats received 3 mEq/kg LiCl, and 20 h later 30 mg/kg pilocarpine. Once SE was achieved, seizures were stopped with 20 mg/kg diazepam. Animals then received 400 mg/kg/day gabapentin or saline for either 4 or 14 days. In vitro experiments were performed in dissociated mixed hippocampal cell culture exposed to glutamate, and subsequently treated with gabapentin or vehicle. During the latency period, the hippocampus and pyriform cortex of SE-animals presented a profuse reactive astrogliosis, with increased GFAP and nestin expression. Gliosis intensity was dependent on the Racine stage attained by the animals and peaked 15 days after SE. Microglia was also reactive after SE, and followed the same pattern. Neuronal degeneration was present in SE-animals, and also depended on the Racine stage and the SE duration. Polysialic-acid NCAM (PSA-NCAM) expression was increased in hippocampal CA-1 and dentate gyrus of SE-animals. Gabapentin treatment was able to reduce reactive gliosis, decrease neuronal loss and normalize PSA-NCAM staining in hippocampal CA-1. In vitro, gabapentin treatment partially prevented the dendritic loss and reactive gliosis caused by glutamate excitotoxicity. Our results show that gabapentin treatment during the latency period after SE protects neurons and normalizes PSA-NCAM probably by direct interaction with neurons and glia.

SUMO-1 conjugation blocks beta-amyloid-induced astrocyte reactivity

Astrocyte reactivity is implicated in the neuronal loss underlying Alzheimer's disease. Curcumin has been shown to reduce astrocyte reactivity, though the exact pathways underlying these effects are incompletely understood. Here we investigated the role of the small ubiquitin-like modifier (SUMO) conjugation in mediating this effect of curcumin. In beta-amyloid (Aβ)-treated astrocytes, morphological changes and increased glial fibrillary acidic protein (GFAP) confirmed reactivity, which was accompanied by c-jun N-terminal kinase activation. Moreover, the levels of SUMO-1 conjugated proteins, as well as the conjugating enzyme, Ubc9, were decreased, with concomitant treatment with curcumin preventing these effects. Increasing SUMOylation in astrocytes, by over-expression of constitutively active SUMO-1, but not its inactive mutant, abrogated Aβ-induced increase in GFAP, suggesting astrocytes require SUMO-1 conjugation to remain non-reactive.

Lactation reduces glial activation induced by excitotoxicity in the rat hippocampus

Motherhood induces a series of adaptations in the physiology of the female, including an increase of maternal brain plasticity and a reduction of cell damage in the hippocampus caused by kainic acid (KA) excitotoxicity. We analysed the role of lactation in glial activation in the hippocampal fields of virgin and lactating rats after i.c.v. application of 100 ng of KA. Immunohistochemical analysis for glial fibrillary acidic protein (GFAP) and ionised calcium binding adaptor molecule 1 (Iba-1), which are markers for astrocytes and microglial cell-surface proteins, respectively, revealed differential cellular responses to KA in lactating and virgin rats. A significant astrocyte and microglial response in hippocampal areas of virgin rats was observed 24 h and 72 h after KA. By contrast, no increase in either GFAP- or Iba-1-positive cells was observed in response to KA in the hippocampus of lactating rats. Western blot analysis of GFAP showed an initial decrease at 24 h after KA treatment, with an increase at 72 h in the whole hippocampus of virgin but not of lactating rats. The number of GFAP-positive cells was increased by lactation in the dentate gyrus of the hippocampus but not in CA1 and CA3 areas. The present results indicate that lactating rats exhibit diminished responses of astrocyte and microglial cells in the hippocampus to damage induced by KA, supporting the notion that the maternal hippocampus is resistant to excitotoxic insults.